The present invention relates to ultra-high resolution imaging devices, including devices using light or other sorts of electromagnetic waves for high resolution lithography for, say, semiconductor or microchip manufacture. The invention involves a combination of an ultra-high numerical aperture imaging system in conjunction with a suitably structured mirror and other associated components designed to result in the boundary conditions to the wave equation arising from the device more nearly approximating to that required to generate an imploding dipole solution to Maxwell""s equations. The invention also provides an ultra-high numerical aperture imaging system using two suitably shaped reflectors, of relatively simple construction which have further potential uses beyond those conventionally relating to ultra-high resolution.
In the context of the present invention xe2x80x9cultra-high resolutionxe2x80x9d means having a resolving power better than that implied by the Rayleigh resolution criterion and xe2x80x9cultra-high numerical aperturexe2x80x9d means that the range of angles that rays make when striking the image plane (if the device is being used to concentrate light) span a high proportion of the total 2xcfx80 it steradians possible for light falling onto one side of a plane. To create sharp images (at least for small objeds), an optical system needs to be aplanatic. Geometrical optical theory indicates that such a system must have at least two surfaces at which the waves are deflected, see e.g. Schulz, G. xe2x80x9cHigher order aplanatismxe2x80x9d, Optics Communications, 41, No 5, 315-319 (1982). The invention provides a two-mirror aplanatic lens arrangement that simultaneously facilitates ultra-high resolution and achieves a very high angle span into a plane.
Attempts to achieve a complete angle span using a combination of a mirror and a refracting surface, rather than two mirrors, have previously been described by Benitez, P. and Mixc3x1ano, J. C. Ultrahigh-numerical-aperture imaging concentrator, J. Opt. Soc. Am. A 14, No 8, 1988-1997 (1997), and in other papers by the same authors. However, the mirror plus refractor arrangement they describe requires the image plane to be embedded within a material with refractive index greater than unity, which is considerably less practical than an approach in which both deflecting surfaces are mirrors.
Benitez and Mixc3x1ano appear to have developed their ideas from non-imaging systems of relatively similar layout that were able to achieve very high concentrations for sources that were not very small. Their imaging layouts are in effect limiting cases of their non-imaging systems when the (far away) source object becomes very small. Other more traditional forms of non-imaging system are known, such as the Compound Parabolic Concentrator (CPC) described in Welford, W. T. and Winston, R. High collection nonimaging optics (Academic Press, 1989). However, the present invention differs from these systems in that it is imaging rather than non-imaging and, as is apparent from a cross-section taken through an axis of symmetry, a device according to the present invention comprises two separate deflecting surfaces not one, as is the case with a CPC. Additionally, in the limit for the CPC as the (far away) source becomes small, the CPC simply becomes arbitrarily long. A device according to the present invention is readily distinguishable.
Some aplanatic two-mirror arrangements have also been previously described. These include:
(a) Siemens-Reiniger-Werke Aktiengesellschaft xe2x80x9cImprovements in or relating to optical mirror systems having aspherical surfacesxe2x80x9d, in GB Patent No GB 0 717 787 (1952). This patent describes a two-mirror aplanalic device, without explicitly specifying any limitation on the numerical aperture involved. However, it does not indicate how to achieve an ultra-high numerical aperture, nor do the Figures that it contains envisage such a device. The patent relates primarily to the design of X-ray telescopes which, because of the physical nature of reflection of X-rays, would not work if the device involved had a very high numerical aperture. Furthermore, despite making reference to a two-mirror aplanatic device, the Siemens-Reiniger-Werke Aktiengesellschaft patent does not indicate how to define the shape of the two mirrors involved.
(b) Mxc3xa4chler, Glxc3xcck, Sclemmer and Bittner xe2x80x9cObjective with aspheric surfaces for imaging microzonesxe2x80x9d, in U.S. Pat. No. 4,655,555 (1984) concentrates on mirrors that use total internal reflection. It includes reference to a special case of an aplanatic two-mirror arrangement involving two confocal equally-sized ellipsoids. It concentrates on other confocal mirror arrangements (as does Hunter xe2x80x9cConfocal reflector systemxe2x80x9d in U.S. Pat. No. 4,357,075 (1980)), although these mirror lay-outs are not actually aplanatic except in the special case of the two confocal equally-sized ellipsoids). However, U.S. Pat. No. 4,655,555 also refers to an article by Lawrence Mertz entitled xe2x80x9cGeometrical Design for Aspheric Reflecting Systemsxe2x80x9d. Applied Optics, 18, pages 4182-4186 (1979), which does appear to describe (in its FIG. 10) a very high numerical aperture aplnatic two-mirror arrangement, again focusing on microscopy. Pioneer xe2x80x9cManufacture of reflective type multiple-degree aspherical optical control systemxe2x80x9d in Japanese Patent JP 57141613 (1981) refers to an efficient means of producing a two mirror aplanatic arranement using a grip and press work plated by aluminium by vapour deposition.
(c) Dxc3x6ring in German Patent DE 2916741 notes that such arrangerrents can be used as optical collectors for solar cells, and the figures suggest reference to aptanatic rather than merely confocal arrangements.
However, none of the above indicate how the precise positioning of the mirrors can be identified. The present invention therefore embodies a significant departure from and advance over the various prior art systems not only because it refers to ultra-high resolution devices but also because it provides a simple methodology for identifying the precise positioning of such aplanatic mirror pairs. In certain preferred embodiments it also incorporates other refinements not described in the above references.
According to one aspect of the present invention a high numerical aperture imaging device comprises first and second axially-symmetric curved mirrors for focussing the image of an object onto an image plane, wherein the first and second curved mirrors are arranged to effectively create inwardly imploding dipole-like solutions to the applicable wave equation, to concentrate the light flux arriving at the image plane from a given point in the object more than would be possible were the image formation to be subject to the diffraction limits that generally apply to far field devices.
In a preferred embodiment, the device further comprises a plane mirror, wherein the plane mirror is partially transparent and is positioned in or closely adjacent to the image plane.
A device according to the invention may further comprise a wave attenuation element and/or wave polarisation-rotating element to attenuate and/or rotate the polarisation of the waves traversing the device so that the spatial distribution of the amplitude and polarisation of a wavefront as it approaches the plane mirror is rendered more closely consistent with that required to generate dipole-like solutions to the wave equation.
Two mirror ultra-high numerical aperture imaging devices according to the invention may have practical application for several possible uses, including, for example:
(a) use to concentrate sunlight to a high temperature, indeed the second law of thermodynamics indicates that the temperatures reached could be dose to the temperature of the sun""s photosphere, i.e. to in excess of 4,000xc2x0 K. At such temperatures, unusual ways of converting sunlight to electric power (e.g. use of thermionic emission) could be facilitated by a device according to the invention;
(b) as solar concentrators made out of lightweight mirrors (for example, using thin films whose shapes remain stable because of rotation [which may require only an initial impetus in a suitable frictionless environment, such as a vacuum, or which could otherwise be achieved with a suitable drive mechanism] or because they are part of an inflated device) such that the power to weight ratio of such an apparatus if used with a lightweight way of converting sunlight to energy could be sufficiently high to permit powered flight (e.g. the sunlight could be used to create direct thrust by evaporation of a solid or liquid propellant);
(c) to concentrate other types of waves such as sound waves or other sorts of electromagnetic radiation like radio waves (e.g. as an alternative to existing parabolic satellite TV dish design);
(d) (when used in reverse) to create narrow beams, e.g. efficient beam formation from light emitted by a light emitting diode in say an optical network;
(e) to create uffra-high resolution imaging devices probably in tandem with additional xe2x80x9cnear fieldxe2x80x9d components;
(f) for concentrating or projecting objects such as gas or dust particles (the trajectories of objects travelling xe2x80x9callisticallyxe2x80x9d are the same as light rays, i.e. straight lines until the object bounces off a surface in the same sort of fashion as a light ray bounces off a mirror, thus the same layouts might also be relevant in the context of such xe2x80x9cballisticxe2x80x9d materials).
In such examples, an ultra high numerical aperture usually provides advantages, e.g. in (a) and (b) it makes it possible to approach more closely the temperature defining the thermodynamic upper limit, in (c) it improves the quality of the signal received for the same aperture area, in (d) it reduces the power required for the same usable energy output and in (e) it ensures that the required boundary conditions can approximate those required to generate an imploding dipole solution to the wave equation. [The xe2x80x9ctraditionalxe2x80x9d parabolic dish achieves about xc2xc of the thermodynamic ideal according to Welford, W. T. and Winston, R. High collection nonimaging optics (Academic Press, 1989).]
Alternative embodiments according to the present invention may include one or more additional focussing mirrors (above two) and/or non-imaging elements for further improving the device to achieve higher order aplanatism.
The invention further provides a method for designing the curved mirrors for use in such high numeric aperture imaging devices.